Review operators in internal interface of ERC777

[frangio]

We've discussed before whether the internal interface of ERC777 should allow arbitrary operator (see _mint for an example). There is some new reasoning to reconsider always setting operator to msg.sender in #2112 (comment).

My feeling is it is more useful to always log msg.sender. Even if the granting was done through logic of the contract, I think for auditing/monitoring purposes it's more relevant to know who triggered it.